Protection element and semiconductor device having the protection element

ABSTRACT

Disclosed herein is a protection element for protecting a circuit element. The protection element includes source and drain areas created in a semiconductor layer, a gate created on the semiconductor layer, sandwiching a gate insulation film between the gate and the semiconductor layer, a source electrode connected to the surface of the source area and electrically connected to the ground, a drain electrode connected to the surface of the drain area and used for receiving a surge input, and a diode connected between the source electrode and the gate.

BACKGROUND

The present disclosure relates to a protection element serving as acountermeasure against ESDs (Electro-Static Discharges) and asemiconductor device provided with the protection element.

As protection elements each serving as a countermeasure against ESDs,there are known a GGMOS (Gate Grounded MOS), a thyristor and an RCtimer, to mention a few.

These protection elements are properly used in their respectiveapplications. The GGMOS has a simple structure so that the GGMOS hasbeen used for the longest period of time among these protectionelements.

A typical structure of the GGMOS is shown in FIG. 7. FIG. 7 is a diagramshowing a GGMOS device of the NMOS type. As shown in the figure, theGGMOS employs a PWell area 51, a source area 52, a drain area 53, a gateinsulation film 54 and a gate 55.

As shown in FIG. 7, in comparison with an ordinary MOS transistor havingthe same structure, in the structure of the GGMOS, the gate 55 isshorted to the source area 52 and both are connected to the ground GNDor the electric potential of the ground.

In the structure of the GGMOS, for a surge input coming from the drain,till an input voltage V represented by the horizontal axis attains acertain voltage level denoted by notation Vt1, the GGMOS does not workas shown in FIG. 8. That is to say, till the input voltage V attains thevoltage level Vt1, a current I represented by the vertical axis does notflow. As the input voltage V attains the voltage level Vt1, a bipolaroperation is started and the input voltage V drops so that the current Ihaving a large magnitude flows.

In the ESD protection element having the related-art GGMOS structure,however, the voltage Vt1 has a specific value due to the GGMOS peculiarconfiguration such as the gate length and impurity concentrations in avariety of areas including the Well, the source and the drain.

Thus, depending on the application, in order to control the voltage Vt1to a desired level, it is necessary to change the configuration of theprotection element.

For raising the voltage Vt1, there are relatively simple methods such asincreasing the gate length.

For reducing the voltage Vt1, there are known the following 3 methods:

-   (1) a method of changing the configuration of the protection element    to an impurity configuration with a low withstanding voltage;-   (2) a method of setting the electric potential of the Well area at a    floated level; and-   (3) a method of providing a circuit for controlling the gate voltage    (refer to M. G. Khazhinskyet al., “Engineering Single NMOS and PMOS    Output Buffers for Maximum Failure Voltage in Advanced CMOS    technologies,” EOS/ESD Symposium 2004, for example, referred to as    Non-Patent Document 1 hereinafter).

As described in Non-Patent Document 1, changes of the voltage Vt1 causedby variations of the gate voltage are used as shown in FIG. 9. Inaddition, a circuit for controlling the gate voltage is provided at aninput stage. This circuit detects a surge current and carries outcontrol so that the gate voltage of the protection element becomes equalto the drain voltage.

Thus, in FIG. 9, Vgs=Vds. As a result, in comparison with therelated-art configuration in which Vgs=0 V, the voltage Vt1 can belowered.

SUMMARY

However, each of the above methods for lowering the voltage Vt1 raisesproblems described as follows.

First of all, in the case of method (1) of changing the configuration ofthe protection element to an impurity configuration with a lowwithstanding voltage, a process of manufacturing a portion of theprotection element is modified.

In addition, if the impurity concentration in the PWell area variesbetween the protection element and the circuit element, it is necessaryto create each of the PWell areas in separate processes, therebyincreasing the number of processes.

In the case of method (2) of setting the electric potential of the Wellarea at a floated level, only two voltages Vt1 can be obtained, that is,the related-art voltage Vt1 and the voltage Vt1 for which the electricpotential of the Well area is set at a floated level. That is to say,only two specific voltage magnitudes, i.e., a high voltage Vt1 and a lowvoltage Vt1 can be implemented.

In the case of method (3) of providing a circuit for controlling thegate voltage, the configuration of the circuit provided at the inputstage to serve as a control circuit becomes complicated and a largerarea is required for the circuit.

In addition, in accordance with the method disclosed in Non-PatentDocument 1 described above, the gate voltage of the protection elementbecomes equal to the drain voltage. Thus, also in the case of thismethod, only two specific voltage magnitudes, i.e., a high voltage Vt1and a low voltage Vt1 can be implemented.

According to an embodiment of the present disclosure, there is provideda protection element, which has a relatively simple configuration and iscapable of setting the voltage Vt1 at any one of three or more levels,to serve as a countermeasure against ESDs (Electro-Static Discharges).There is also provided a semiconductor device having the protectionelement.

The protection element according to the embodiment of the presentdisclosure is a protection element for protecting a circuit element.

The protection element includes source and drain areas created in asemiconductor layer; a gate created on the semiconductor layer,sandwiching a gate insulation film between the gate and thesemiconductor layer; a source electrode connected to the surface of thesource area and electrically connected to the ground; a drain electrodeconnected to the surface of the drain area and used for receiving asurge input; and a diode connected between the source electrode and thegate.

A semiconductor device according to another embodiment of the presentdisclosure includes a circuit element and a protection element which hasthe configuration of the protection element described above and isconnected to the circuit element.

In the configuration of the protection element according to theembodiment of the present disclosure, the diode is connected between thegate and the source electrode electrically connected to the ground.Thus, an electric potential appearing on the gate can be shifted fromthe ground so that the magnitude of the voltage Vt1 explained earliercan be changed.

In addition, according to the number of diodes connected between thegate and the source electrode, the electric potential appearing on thegate can be changed so that the magnitude of the voltage Vt1 can bechanged in accordance with the change of the electric potential.

In the configuration of the semiconductor device according to theembodiment of the present disclosure, the protection element accordingto the embodiment of the present disclosure is connected to a circuitelement. Thus, in the protection element, in accordance with the numberof diodes connected between the gate and the source electrode, theelectric potential appearing on the gate can be changed so that themagnitude of the voltage Vt1 can also be changed.

In the embodiments of the present disclosure described above, thevoltage Vt1 can be changed according to the number of diodes connectedbetween the gate and the source electrode. Thus, it is possible to setthe voltage Vt1 at any one of three or more levels.

In addition, in comparison with the protection element having therelated-art GGMOS configuration, it is also possible to make aprotection element having a lower voltage Vt1.

The protection element according to the embodiment of the presentdisclosure has a configuration including a diode within the MOSstructure. Thus, the diode can be embedded in the MOS structure duringmanufacturing the structure. As a result, since no additional process isrequired, the protection element can be made with ease in the process ofmanufacturing an ordinary MOS structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a rough diagram showing a cross section of the configurationof a protection element according to a first embodiment of the presentdisclosure;

FIG. 2 is a diagram showing results computed by the TCAD (TechnologyComputer Aided Design) simulation regarding relations between the drainvoltage and a current when the number of diodes varies in theconfiguration shown in FIG. 1;

FIG. 3 is a rough diagram showing a cross section of a configuration ofa protection element according to a second embodiment of the presentdisclosure;

FIG. 4 is a rough diagram showing a cross section of a configuration ofa protection element according to a third embodiment of the presentdisclosure;

FIG. 5 is a diagram showing results computed by the TCAD simulationregarding relations between the drain voltage and a current when thenumber of diodes varies in the configuration shown in FIG. 4;

FIG. 6 is a rough diagram showing a cross section of a configuration ofa protection element according to a fourth embodiment of the presentdisclosure;

FIG. 7 is a cross section of a protection element having the related-artGGMOS structure;

FIG. 8 is a diagram showing a relation between a surge input voltage anda leak current in the GGMOS structure; and

FIG. 9 is a diagram showing relations between the surge input voltageand the leak current for different magnitudes of the gate voltage in theGGMOS structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present disclosure are explained below. Inthe following description, each of the preferred embodiments is alsoreferred to simply as an embodiment.

It is to be noted that the embodiments are explained in chaptersarranged as follows:

-   1: First Embodiment-   2: Second Embodiment-   3: Third Embodiment-   4: Fourth Embodiment-   5: Modifications

1. First Embodiment

FIG. 1 is a rough diagram showing a cross section of the configurationof a protection element according to a first embodiment of the presentdisclosure.

Much like the protection element having the GGMOS structure shown inFIG. 7, the protection element according to the first embodiment is aprotection element used as a countermeasure against ESDs (Electro-StaticDischarges).

As shown in FIG. 1, on the surface portion of a PWell area 11 created ona semiconductor layer such as a semiconductor substrate or an epitaxiallayer, a source area 12 and a drain area 13 are created as n⁺ impurityareas. Then, above the semiconductor layer between the source area 12and the drain area 13, a gate 15 is created to sandwich a gateinsulation film 14 between the semiconductor layer and the gate 15.

That is to say, the protection element has the same NMOS structure as anordinary NMOS transistor.

Each of the source area 12 and the drain area 13 is connected to anelectrode 16 provided on the surface of the semiconductor layer. Inaddition, portions above the electrodes 16 and portions outside the gate15 are covered with an insulation layer 17.

In the protection element according to this embodiment, the electrode(source electrode) 16 connected to the source area 12 is also connectedto the gate 15. Unlike the configuration shown in FIG. 7 in which thesource electrode is electrically connected directly to the gate, thesource electrode 16 is particularly connected to the gate 15 through twodiodes 21 (21A and 21B).

The diodes 21A and 21B are connected in series between the gate 15 andthe ground GND or a ground electric potential with their forwarddirections oriented toward the ground.

It is to be noted that, in the configuration described above, the sourceelectrode 16 connected to the source area 12 is directly connected tothe ground as is the case with the configuration shown in FIG. 7.

Thus, an electric potential appearing on the gate 15, which is equal toan electric potential A in FIG. 1, can be shifted from the ground in thepositive direction by a voltage difference equal to a voltage drop alongthe diodes 21A and 21B.

Since the electric potential appearing on the gate 15 can be shiftedfrom the ground electric potential in the positive direction in thisway, the voltage Vt1 can be reduced as is obvious from FIG. 9 explainedearlier.

In addition, in the protection element according to this embodiment, thedrain electrode 16 of the drain area 13 is, as an electrode forreceiving a surge input, electrically connected to the gate 15 through aresistor 22.

With the drain electrode 16 electrically connected to the gate 15through the resistor 22, the surge input is also supplied to the gate15.

In addition, by electrically connecting the drain electrode 16 to thegate 15 through the resistor 22, the electric potential A supplied tothe gate 15 can be made lower than the voltage of the surge input by avoltage drop along the resistor 22 in comparison with a configurationnot including the resistor 22.

It is to be noted that, in the configuration not including the resistor22, the resistance on the side of the diodes 21A and 21B becomes smallso that most of the current flows to the diodes 21A and 21B. As aresult, current does not flow between the source area 12 and the drainarea 13, causing a snapback operation to be no longer carried out. Ifthe number of diodes 21 is increased in order to make it difficult forthe current to flow to the diodes 21, the snapback operation can becarried out. In such a configuration including more diodes 21, however,the electric potential appearing on the gate 15 becomes equal to theelectric potential appearing on the drain area 13 unless the resistor 22is provided. Thus, the voltage Vt1 can be set only at a fixed magnitudeas is obvious from FIG. 9.

Each of the diodes has a simple structure which is a combination of Pand N areas. When a MOS structure is made, in general, the diode canalso be created at the same time as the MOS structure.

By the same token, in the process of making a MOS structure, a resistorcan also be created at the same time as the MOS structure.

That is to say, in the protection element according to this embodiment,the diodes 21 (21A and 21B) as well as the resistor 22 can be created inother portions of the semiconductor layer. These other portions areportions separated from portions used for creating the PWell area 11,the source area 12 and the drain area 13 which are included in the NMOSstructure of the protection element.

Thus, it is possible to create the diodes 21 (21A and 21B) as well asthe resistor 22 without adding a special process to a process ofmanufacturing the portions of the NMOS structure of the protectionelement and manufacturing circuit elements provided with the protectionelement.

In particular, if the impurity concentrations of the P and N areas ofthe diodes 21 (21A and 21B) are the same as the impurity concentrationof the circuit element or the impurity concentration of the PWell area11, the source area 12, the drain area 13 or the like which are includedin the protection element, the diodes 21 can be created at the same timeas the component having the same impurity concentration.

Next, operations carried out by the protection element according to thisembodiment are explained as follows.

First of all, when a high voltage is supplied to the protection elementas a surge input, the high voltage is applied to the drain area 13 andthe resistor 22 which are included in the NMOS structure. Thus, avoltage equal to or higher than the threshold voltage of the diodes 21Aand 21B is applied to each of the diodes 21A and 21B, putting the diodes21A and 21B in a conductive state. In the case of a siliconsemiconductor layer, the threshold voltage is about 0.7 V.

At that time, the electric potential A shown in FIG. 1 is set at a levelhigher than the ground by an electric-potential drop along the diodes21A and 21B. The electric-potential drop is calculated by multiplying0.7 V by the number of diodes. For example, the electric potential A issustained at 0.7 V when one diode is used and 1.4 V when two diodes areused like FIG. 1.

As is also obvious from FIG. 9 explained before, if the electricpotential appearing on the gate 15 is raised from 0 V, the voltage Vt1once decreases. As the electric potential appearing on the gate 15 isfurther raised, the voltage Vt1 conversely increases to a level lowerthan the level of the voltage Vt1 which is obtained when the electricpotential appearing on the gate is set at 0 V.

In this way, the voltage Vt1 can be controlled in accordance with thenumber of diodes connected between the gate 15 and the ground.

If the gate is simply shorted to the drain, the voltage Vt1 becomeslower than that of the GGMOS and can be set only at one magnitude sothat the voltage Vt1 cannot be controlled in a way according to thepresent disclosure.

It is to be noted that the resistance of the resistor 22 can be set witha high degree of freedom at any value as long as the value is smallerthan the resistance when the diodes 21A and 21B are put in anonconductive state but greater than a smallest possible resistance atwhich a current flows between the source area 12 and the drain area 13.The resistance when the diodes 21A and 21B are in a nonconductive statehas an extremely high value.

In this case, a TCAD (Technology CAD) simulation has been carried out inorder to predict operations which are carried out by the protectionelement according to this embodiment when a surge input is supplied tothe protection element.

To put it more concretely, the simulation has been carried out for astructure including two diodes connected between the gate and the sourceas shown in FIG. 1 and a structure including one diode connected betweenthe gate and the source. In addition, the same operations have also beenpredicted for the related-art GGMOS structure serving as a comparisonstructure.

FIG. 2 is a diagram showing the results representing relations betweenthe drain voltage and a current flowing between the drain and the sourcefor each of the structures.

As is obvious from the results shown in FIG. 2, in the case of therelated-art GGMOS structure, the voltage Vt1 is 8.8 V. In the case ofthe structure including one diode in accordance with this embodiment,the voltage Vt1 is decreased to 5.7 V. In the case of the structureincluding two diodes in accordance with this embodiment, the voltage Vt1is decreased to 4.8 V.

That is to say, it is confirmed possible to control the magnitude of thevoltage Vt1 in accordance with the number of diodes.

In the configuration of the protection element according to theembodiment, the gate 15 is connected to the source electrode 16 of thesource area 12 through the two diodes 21A and 21B.

Thus, since the electric potential A appearing on the gate 15 can beshifted from the ground in the positive direction, the voltage Vt1 canbe reduced.

In addition, the number of diodes is not limited to two as is the casewith the configuration shown in FIG. 1. For example, the number ofdiodes can also be one or three or an integer greater than 3. Bychanging the number of diodes, it is possible to change the electricpotential appearing on the gate 15 and, hence, the voltage Vt1.

Since the voltage Vt1 can be changed by changing the number of diodes asdescribed above, it is possible to set the voltage Vt1 at any one ofthree or more levels. In the case of the results shown in FIG. 2 forexample, the voltage Vt1 can be set at 8.8 V, 5.7 V or 4.8 V.

In addition, in the configuration of the protection element according tothe embodiment, through the resistor 22, the gate 15 is electricallyconnected to the drain electrode 16 of the drain area 13 receiving thesurge input.

Thus, the electric potential A appearing on the gate 15 can be madelower than the voltage of the surge input by a voltage difference equalto a voltage drop along the resistor 22.

In the protection element according to this embodiment, the diodes 21and the resistor 22 are merely added to the NMOS structure so that thediodes 21 and the resistor 22 can be embedded in the MOS structureduring a process of manufacturing the structure.

That is to say, the protection element according to this embodiment doesnot require an additional process and can thus be created with ease bycarrying out the ordinary process of making an MOS structure.

By making use of the protection element according to this embodiment, itis possible to create a semiconductor device having the protectionelement.

For example, in the periphery of a circuit element configuring thesemiconductor device, the protection element according to thisembodiment is provided as an element for receiving a surge input.

2. Second Embodiment

In the configuration of the first embodiment shown in FIG. 1, in anormal operating state where a voltage Vdd around the range from 1.8 Vto 5 V is applied to the drain area 13, the diodes 21 conduct, settingthe electric potential A shown in the figure at a level calculated bymultiplying 0.7 V by the number of diodes.

At that time, the channel of the MOS structure is opened, allowing acurrent to leak from the drain area 13 to the source area 12. Thus, anelectric power corresponding to the leak current is consumed.

A configuration for preventing the leak current from flowing isimplemented as a second embodiment described as follows.

FIG. 3 is a rough diagram showing a cross section of a configuration ofa protection element according to the second embodiment of the presentdisclosure.

As shown in the figure, in particular, this embodiment has aconfiguration in which diodes 23 and a resistor 24 are provided inaddition to the diodes 21 and the resistor 22 shown in FIG. 1.

The diodes 23 are connected in series between the drain electrode 16 ofthe drain area 13 and the resistor 22 with their forward directionsoriented toward the ground in the same way as the two diodes 21A and21B. The diodes 23 include four diodes 23A, 23B, 23C and 23D.

The resistor 24 is connected between the gate 15 and the sourceelectrode 16 of the source area 12 in parallel to the diodes 21.

In FIG. 3, the diodes 21 include the two diodes 21A and 21B, and thediodes 23 include the four diodes 23A, 23B, 23C and 23D. It is to benoted, however, that the number of diodes 21 and 23 can be changed ifnecessary.

In addition, the diodes 23 are provided on the surge-input side of theresistor 22 in FIG. 3. However, it is possible to provide aconfiguration in which the resistor 22 is conversely provided on thesurge-input side of the diodes 23.

Next, operations carried out by the protection element according to thisembodiment are explained as follows.

In a normal operating state in which the drain voltage is set at thepower-supply voltage Vdd of 2.7 V, the drain voltage is lower than thethreshold voltage (0.7 V×4 diodes=2.8 V) for the diodes 23. Thus, thediodes 23 do not conduct. At that time, the gate electric potential is 0V. Thus, the channel is closed and no leak current flows.

As the drain voltage further increases, the diodes 23 conduct, andthereby a current flows through a route including the diodes 23, theresistor 22, the resistor 24 and the ground. At that time, the gatevoltage increases to a level of I×R2, where notation I denotes themagnitude of the current and notation R2 denotes the resistance of theresistor 24.

As the current I further increases, the gate voltage I×R2 puts thediodes 21 in a conductive state.

The electric potential appearing at that time on the gate 15 is the gatevoltage putting the diodes 21 in a conductive state, that is 0.7 V×2diodes=1.4 V in the case of FIG. 3.

It is to be noted that, even if the drain voltage further increases, thegate electric potential is sustained at 1.4 V.

Even if the operating voltage is a voltage other than 2.7 V, therequired number of diodes of the diodes 23 can be determined so that thetotal threshold voltage of the diodes 23 is higher than the operatingvoltage.

As described above, the diodes 23 connected between the drain and thegate 15 set the gate electric potential at 0 V in a state in which thedrain voltage is lower than the voltage at which the diodes 23 canconduct. Thus, in this state, it is possible to prevent a leak currentfrom flowing between the source area 12 and the drain area 13.

Since the other components of the configuration are identical with thoseof the first embodiment shown in FIG. 1, the other components aredenoted by the same reference numerals as the first embodiment and it isthus abbreviated to again explain the other components.

In the configuration of the protection element according to theembodiment, the gate 15 is connected to the source electrode 16 of thesource area 12 through the two diodes 21 (21A and 21B) and the resistor24.

Thus, in the same way as the first embodiment, the electric potential Aof the gate 15 can be shifted to a positive electric potential from theground. As a result, the voltage Vt1 can be lowered.

In addition, the number of diodes 21 is not limited to two as shown inFIG. 3. For example, the number of diodes 21 can also be one or three oran integer greater than three. By changing the number of diodes, it ispossible to change the electric potential appearing on the gate 15 and,hence, the voltage Vt1.

Since the voltage Vt1 can be changed by changing the number of diodes asdescribed above, it is possible to set the voltage Vt1 at any one ofthree or more levels according to the number of diodes.

In addition, in the configuration of the protection element according tothe embodiment, through the resistor 22 as well as the diodes 23A, 23B,23C and 23D, the gate 15 is electrically connected to the drainelectrode 16 of the drain area 13 for receiving the surge input.

Thus, the electric potential A appearing on the gate 15 can be madelower than the voltage of the surge input.

In the protection element according to this embodiment, the diodes 21,the diodes 23, the resistor 22 and the resistor 24 are merely added tothe NMOS structure so that the diodes 21, the diodes 23, the resistor 22and the resistor 24 can be embedded in the MOS structure during aprocess of manufacturing the structure.

That is to say, the protection element according to this embodiment doesnot require an additional process and can thus be created with ease bycarrying out the ordinary process of making an MOS structure.

In addition, in the protection element according to this embodiment, thediodes 23 are connected between the resistor 22 and the surge input, andthe resistor 24 is connected in parallel to the diodes 21. Thus, theelectric potential on the gate 15 remains at 0 V in a state in which theoperating voltage is lower than the total threshold voltage of thediodes 23. As a result, in this state, it is possible to prevent a leakcurrent from flowing between the source area 12 and the drain area 13and hence to prevent electric power from being consumed due to a flowingleak current.

In addition, by making use of the protection element according to thisembodiment, it is possible to create a semiconductor device having theprotection element.

For example, in the periphery of a circuit element configuring thesemiconductor device, the protection element according to thisembodiment is provided as an element for receiving a surge input.

3. Third Embodiment

FIG. 4 is a rough diagram showing a cross section of a configuration ofa protection element according to a third embodiment of the presentdisclosure.

In general, when a high voltage is applied to a MOS drain, the voltageof the gate also rises as well.

In the embodiment, this phenomenon is used as follows. As shown in FIG.4, the gate 15 is not electrically connected to the drain. The gate 15is connected only to the ground through the diodes 21 (21A and 21B).

By configuring this embodiment as described above, when a surge input isapplied to the drain, the electric potential appearing on the gate 15makes an attempt to rise but the electric potential is clamped at alevel by the diodes 21 (21A and 21B). Thus, the electric potentialappearing on the gate 15 is prevented from rising beyond the clampedlevel.

Since the other components of the configuration are identical with thoseof the first embodiment shown in FIG. 1, the other components aredenoted by the same reference numerals as the first embodiment and it isthus abbreviated to again explain the other components.

In this case, a TCAD (Technology CAD) simulation has been carried out inorder to predict operations which are carried out by the protectionelement according to this embodiment when a surge input is supplied tothe protection element.

To put it more concretely, the simulation has been carried out for astructure including two diodes connected between the gate and the sourceas shown in FIG. 4 and a structure including one diode connected betweenthe gate and the source. In addition, the same operations have also beenpredicted for a structure in which the number of diodes is extremelylarge to practically result in a gate-open state.

FIG. 5 is a diagram showing the results representing relations betweenthe drain voltage and a current flowing between the drain and the sourcefor each of the structures.

As is obvious from the results shown in FIG. 5, the voltage Vt1 variesin accordance with the number of diodes. The voltage Vt1 of therelated-art GGMOS structure is 8.8 V as shown in FIG. 2. In the case ofthe structure including one diode in accordance with this embodiment,the voltage Vt1 is 6.8 V. In the case of the structure including twodiodes in accordance with this embodiment, the voltage Vt1 is 6.2 V. TheVt1 values of these two cases in this embodiment are closer to thevoltage Vt1 of 5.3 V in the gate-open state, when comparing to thevoltage Vt1 of the GGMOS structure shown in FIG. 2.

In comparison with the first embodiment shown in FIG. 1 and the secondembodiment shown in FIG. 3, the Vt1 quantity controlled by the changesin the number of diodes decreases in the configuration of the thirdembodiment. However, the third embodiment offers a merit that, by makinguse of a similar circuit, it is possible to prevent a leak current frombeing generated. In addition, since the Vt1 quantity controlled by thechanges in the number of diodes decreases, the third embodiment is moreappropriate for an application in which the voltage Vt1 is to be finelycontrolled.

In the configuration of the protection element according to theembodiment, the gate 15 is connected to the source electrode 16 of thesource area 12 through the two diodes 21 (21A and 21B).

Thus, in the same way as the first embodiment, the electric potential Aof the gate 15 can be shifted to a positive electric potential from theground and the voltage Vt1 can therefore be reduced.

In addition, the number of diodes 21 is by not limited to two as shownin FIG. 4. For example, the number of diodes 21 can also be one or threeor an integer greater than three. By changing the number of diodes, itis possible to change the electric potential appearing on the gate 15and, hence, the voltage Vt1.

Since the voltage Vt1 can be changed by changing the number of diodes asdescribed above, it is possible to set the voltage Vt1 at any one ofthree or more levels according to the number of diodes.

In addition, in the configuration of the protection element according tothe embodiment, the gate 15 is not electrically connected to the drainelectrode 16 of the drain area 13 which receives the surge input.

Thus, the electric potential A appearing on the gate 15 makes an attemptto rise corresponding to the voltage rise in the surge input, but theelectric potential is clamped at a certain level by the diodes 21. As aresult, the electric potential appearing on the gate 15 is preventedfrom rising beyond the certain level.

In the protection element according to this embodiment, the diodes 21are merely added to the NMOS structure so that the diodes 21 can beembedded in the MOS structure during a process of manufacturing thestructure.

That is to say, the protection element according to this embodiment doesnot require an additional process and can thus be created with ease bycarrying out the ordinary process of making an MOS structure.

By making use of the protection element according to this embodiment, itis possible to create a semiconductor device having the protectionelement.

For example, in the periphery of a circuit element configuring thesemiconductor device, the protection element according to thisembodiment is provided as an element for receiving a surge input.

4. Fourth Embodiment

FIG. 6 is a rough diagram showing a cross section of a configuration ofa protection element according to a fourth embodiment of the presentdisclosure.

In this embodiment, as shown in FIG. 6, the protection element includestwo NMOS structures which are each identical with that of the firstembodiment shown in FIG. 1. The diodes 21 and resistor 22 are providedas components common to the two NMOS structures.

In this way, by making use of a set including diodes 21 and a resistor22, a plurality of MOS gates can be controlled. Thus, the configurationof the fourth embodiment is simpler than a configuration in which thegates of a plurality of protection elements are each provided with a setincluding of diodes 21 and a resistor 22.

In the configuration shown in FIG. 6, two MOS gates are controlled bymaking use of a set including diodes 21 and a resistor 22. It is to benoted, however, that, a set including diodes 21 and a resistor 22 can beused for controlling three or more MOS gates.

Since the other components of the configuration are identical with thoseof the first embodiment shown in FIG. 1, the other components aredenoted by the same reference numerals as the first embodiment and it isthus abbreviated to again explain the other components.

In the configuration of the protection element according to theembodiment, the gate 15 is connected to the source electrode 16 of thesource area 12 through the two diodes 21 (21A and 21B).

Thus, in the same way as the first embodiment, the electric potential Aof the gate 15 can be shifted to a positive electric potential from theground, and the voltage Vt1 can therefore be reduced.

In addition, the number of diodes is not limited to two as shown in FIG.6. For example, the number of diodes can also be one or three or aninteger greater than three. By changing the number of diodes, it ispossible to change the electric potential appearing on the gate 15 and,hence, the voltage Vt1.

Since the voltage Vt1 can be changed by changing the number of diodes asdescribed above, it is possible to set the voltage Vt1 at any one ofthree or more levels according to the number of diodes.

In addition, in the configuration of the protection element according tothe embodiment, the gate 15 is electrically connected to the drainelectrode 16 of the drain area 13 which receives the surge input,through the resistor 22.

Thus, the electric potential A appearing on the gate 15 can be madelower than the voltage of the surge input in the same way as the firstembodiment.

In the protection element according to this embodiment, the diodes 21and the resistor 22 are merely added to the NMOS structure so that thediodes 21 and the resistor 22 can be embedded in the MOS structureduring a process of manufacturing the structure.

That is to say, the protection element according to this embodiment doesnot require an additional process and can thus be created with ease bycarrying out the ordinary process of making an MOS structure.

By making use of the protection element according to this embodiment, itis possible to create a semiconductor device having the protectionelement.

For example, in the periphery of a circuit element configuring thesemiconductor device, the protection element according to thisembodiment is provided as an element for receiving a surge input.

In FIG. 6, the configurations of the diodes 21 and the resistor 22 aremade identical with those of the first embodiment shown in FIG. 1.

The configurations of diodes and resistors can also be made identicalwith those of the second embodiment shown in FIG. 3 or the thirdembodiment shown in FIG. 4 so that a plurality of MOS gates can becontrolled.

5. Modifications

Each of the embodiments described above applies the present disclosureto an NMOS structure. However, the present disclosure can also beapplied to a PMOS structure.

In an application of the present disclosure to a PMOS structure, diodesare provided between the gate and the source area in the same way as anNMOS structure. In the PMOS structure, however, a negative voltage isapplied to the gate. Thus, the forward directions of the diodes providedbetween the gate and the source area in the PMOS structure are orientedin a direction opposite to the direction in the NMOS structure. Thedirection opposite to the direction in the NMOS structure is a directionfrom the ground to the gate.

In addition, in a semiconductor device wherein a protection element isconnected to a circuit element, it is possible to provide aconfiguration in which a plurality of protection elements are providedand the protection elements include two or more protection elementshaving voltages Vt1 different from each other.

In this case, the two or more protection elements having voltages Vt1different from each other can be made by changing the number of diodesincluded in each of the protection elements. It is not necessary to addprocesses in order to separately create the MOS structures havingvoltages Vt1 different from each other. Thus, the manufacturing time andcost can be cut down.

It is to be noted that, when configuring a semiconductor device bymaking use of a protection element according to any of the embodimentsdescribed above, the protection element can be connected to a circuitelement of the semiconductor device in the same way as the connectionbetween a protection element having the related-art GGMOS structure anda circuit element. For example, the drain side of the protection elementis connected to the front of a circuit element of a wire conveying asurge input.

The present disclosure contains subject matter related to that disclosedin Japanese Priority Patent Application JP 2011-000806 filed in theJapan Patent Office on Jan. 5, 2011, the entire content of which ishereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1. A protection element for protecting a circuit element, saidprotection element comprising: source and drain areas created in asemiconductor layer; a gate created on said semiconductor layer,sandwiching a gate insulation film between said gate and saidsemiconductor layer; a source electrode connected to the surface of saidsource area and electrically connected to the ground; a drain electrodeconnected to the surface of said drain area and used for receiving asurge input; and a diode connected between said source electrode andsaid gate.
 2. The protection element according to claim 1, wherein aplurality of said diodes is connected in series orienting the forwarddirections of said diodes in the same direction.
 3. The protectionelement according to claim 1, said protection element further includinga resistor connected between said drain electrode and said gate.
 4. Theprotection element according to claim 3, said protection element furtherincluding a second resistor connected in parallel to said diode andfurther including a second diode connected in series with said resistor.5. The protection element according to claim 1, wherein said diode isconnected to a plurality of sets including said gate and said sourceelectrode to serve as a diode common to said sets.
 6. A semiconductordevice comprising a circuit element and a protection element forprotecting said circuit element and provided with: source and drainareas connected to said circuit element and created in a semiconductorlayer; a gate created on said semiconductor layer, sandwiching a gateinsulation film between said gate and said semiconductor layer; a sourceelectrode connected to the surface of said source area and electricallyconnected to the ground; a drain electrode connected to the surface ofsaid drain area and used for receiving a surge input; and a diodeconnected between said source electrode and said gate.
 7. Thesemiconductor device according to claim 6, wherein said protectionelement further includes a resistor connected between said drainelectrode and said gate.
 8. The semiconductor device according to claim7, wherein said protection element further includes a second resistorconnected in parallel to said diode and further includes a second diodeconnected in series with said resistor.